IOTA - the Infrared-Optical Telescope Array

A Bit of History

IOTA began with an agreement in 1988 among five Institutions, the Smithsonian
Astrophysical Observatory, Harvard University, the University of Massachusetts,
the University of Wyoming, and MIT/Lincoln Laboratory, to build a two-telescope
stellar interferometer for the purpose of making fundamental astrophysical
observations, and also as a prototype instrument on which we could perfect
techniques which could later lead to the development of a larger, more
powerful array. On site construction went on for all 1993 and 1994, with
first fringes in December 1993.

Milestones

Some of the most important milestones in the technology development
of the IOTA stellar interferometer and the growing scientific capabilities
that this has implied are presented in the following table.

In the past recent years the membership has evolved as the location
of participant scientists have changed, so that currently SAO, Harvard,
and UMass are active from the original five, and six new institutional
affiliation have begun: Instituto Nacional de Astrofisica, Optica y Electronica
(INAOE), Observatoire de Grenoble, Observatoire de Paris- Meudon, NASA
Ames Research Center, ESO (Garching) and MPIA (Heidelberg), the last
two participating via individual scientists.

The Instrument

The IOTA, is a Michelson stellar interferometer located on Mt. Hopkins
in southern Arizona. It operates with three 45 cm collectors
that can be located at different stations on each arm of an L-shaped array
(15 m X 35 m) and reaches a maximum baseline of 38 m.
Each light collector consists of a siderostat feeding a stationary afocal
Cassegrain telescope which produces a 10X reduced parallel beam. After
compression each beam is directed vertically downward by a piezo-driven
active mirror that corrects for tip-tilt motion introduced by atmospheric
turbulence. The beams then enter an evacuated envelope and proceed to the
corner of the array, where they are turned back along one arm for path
compensation, which occurs by means of the delay lines. After being delayed
the beams exit from the vacuum and are directed into the designated combination
and detection area.

There are three combination tables at the IOTA, and all of them
implement pupil-plane beam combination. In two cases, at visible and near-IR
wavelengths, the combination occurs at a beam splitter. The third
table houses the FLUOR experiment, in which beam combination, also at near-IR
wavelengths, occurs in single-mode fibers. For near-IR operation,
a pair of dichroic mirrors at 45° transmit wavelengths less than
1 micron toward the CCD based tip-tilt servo system, and reflect the near-IR
light toward the beam combining optics and science detector. The near-IR
detector is based on a 256x256 HgCdTe NICMOS3 array. For visible operation
only part of the visible light is directed towards the star tracker CCD,
and most of the light is directed towards the visible table for combination.
The IOTA visible light detector is a thinned back-side-illuminated SITe
512x512 CCD.